I saw my first provider not long ago, and would like to see here again. I'd like to book for 90 minutes, but her website only lists rates by the hour. So how do I ask about a 90min rate without offending her by mentioning money? Thanks in adavance for the help.

Just send her an e-mail stating how much you enjoyed your time together but felt it wasn't long enough, and what would her rate be for 90 minutes of her time.

Nothing explicit, you are just paying for her time...